White polyimide film and manufacture thereof

ABSTRACT

A white polyimide film comprises a polyimide base polymer and a coloration filler homogeneously distributed in the white polyimide film by reacting a diamine component with a dianhydride component and a coloration filter. The diamine component can include 2,2′-bis(trifluoromethyl)benzidine, and the dianhydride component can include 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride. The coloration filler can include TiO2, Al2O3, CaCO3, CaSO4, SiO2, BN, AlN and clay. The white polyimide film has a coefficient of thermal expansion (CTE) less than about 60 ppm/° C. in a temperature range between about 100° C. and about 200° C. , an elongation rate between about 5% and about 60%, and a b*-value between about 0 and about 15. In some embodiments, a method of manufacturing the white polyimide film is also described.

BACKGROUND

1. Field of the Invention

The present invention relates to the manufacture of polyimide films, andmore particularly to color polyimide films such as white polyimide.

2. Description of the Related Art

Polyimide (PI) films are lightweight, flexible, and exhibit excellentmechanical properties as well as good resistance to heat and chemicals.Therefore, PI films are widely used in the electronics industry, such asin the fabrication of light-emitting diode (LED) devices, liquid crystaldisplay devices, and the like. However, as the electronic industryadvances, more demands are put on the quality and characteristics of PIfilms. In particular, certain design requirements may impose specificfilm coloration, e.g., opaque white, which is different from theconventional orange/yellow color characteristic of the PI film.

To form a white polyimide film, one known approach consists in coating awhite resin (e.g., epoxy resin, acrylic resin or polydimethylsiloxaneresin) on the conventional polyimide film for forming a dual-layeredpolyimide film. While this approach can form a polyimide film with thedesired white tint, coating additional resin usually increases themanufacture cost and adversely affects film properties. In particular,because the resin coating generally has poor thermal resistance, colordeterioration or yellowing can be usually observed when the PI film isexposed to thermal stress.

Another known approach consists in incorporating a white filler duringthe process of forming the polyimide film. In this manner, a polyimidefilm can be formed with the desired white color without the need ofadditional resin coating. This alternative approach typically adopts a“one-step method”; i.e., the condensation polymerization andring-closing reaction of diamine monomers and dianhydride monomers arecompleted in one reactor, and a solution containing polyimide isdirectly obtained. The solvent of the polyimide solution can be thenremoved to obtain a solid polyimide film. Unfortunately, white polyimidefilms formed with this method still have insufficient resistance tochemicals and do not meet the design requirements for LED manufacture.

Therefore, there is a need for a white polyimide film that can haveenhanced properties and can be manufactured in a cost-effective manner.

SUMMARY

The present application describes a white polyimide film and its methodof manufacture. The white polyimide film can be prepared by reacting adiamine component with a dianhydride component. More particularly, oneembodiment of the white polyimide film can comprise a polyimide basepolymer that has a combination of molecular structures represented bythe following formulae (I) and (II):

-   -   wherein the m:n ratio representing the molar ratio of the molar        structure (I) and (II) in the polyimide base polymer is about        0.95-0.05:0.05-0.95;    -   and a coloration filler homogeneously distributed in the white        polyimide film.

The present application also provides a white polyimide film comprising:a polyimide base polymer and a coloration filler, wherein the whitepolyimide film has a coefficient of thermal expansion (CTE) less thanabout 60 ppm/° C. in a temperature range between about 100° C. and about200° C., an elongation rate between about 5% and about 60%, a L*-valuemore than about 90, and a b*-value between about 0 and about 15.

In other embodiments, the present application also provides a process ofmanufacturing a white polyimide film, comprising performing condensationpolymerization of monomers comprising diamine and dianhydride componentsto obtain a polyamic acid solution, adding a dehydrant, a catalyst and acoloration filler into the polyamic acid solution to obtain a precursorsolution, coating a layer of the precursor solution on a supportingbase, and baking the coated layer to form a white polyimide film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating method steps for forming a whitepolyimide film.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present application provides a white polyimide film with enhancedproperties and its method of manufacture. The polyimide film is formedas a single white-colored layer, comprising:

-   -   a polyimide base polymer obtained by reacting a diamine        component with a dianhydride component, the polyimide base        polymer consisting of a combination of molecular structures        represented by the following formulae (I) and (II):

-   -   wherein the m:n ratio representing the molar ratio of the molar        structure (I) and (II) in the polyimide base polymer is about        0.95-0.05:0.05-0.95; and    -   a coloration filler homogeneously distributed in the polyimide        film.

The m:n ratio can be between about 0.95-0.05:0.05-0.95, such as betweenabout 0.85-0.20:0.15-0.80, for example 0.80-0.40:0.20-0.60. Statedanother way, the polyimide base can comprise about 95 mol % to about 5mol % of molar structure (I) (m) and about 5 mol % to about 95 mol % ofmolar structure (II) (n), such as about 85 mol % to about 15 mol % of mand about 20 mol % to 80 mol % of n, for example about 80 mol % to about20 mol % of m and about 40 mol % to about 60 mol % of n.

In one embodiment, examples of suitable diamine components can include2,2′-bis(trifluoromethyl) benzidine (TFMB) represented by the followingformula:

Examples of suitable dianhydride components can be selected from thegroup consisting of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA),respectively represented by the following formulae:

In the white polyimide film, the coloration filler is homogeneouslymixed with the polyimide base polymer. The coloration filler can beselected from the group consisting of titanium dioxide (TiO₂), zirconiumoxide (ZrO₂), calcium oxide (CaO), zinc oxide (ZnO₂), aluminum oxide(Al₂O₃), zinc sulfide (ZnS₂), calcium carbonate (CaCO₃), lead carbonate(PbCO₃), lead hydroxide (Pb(OH)₂), calcium sulfate (CaSO₄), bariumsulfate (BaSO₄), silicon dioxide (SiO₂), boron nitride (BN), aluminumnitride (AlN), basic zinc molybdate, basic calcium zinc molybdate, leadwhite, molybdenum white, lithopone (a mixture of barium sulfate and zincsulfide), and clay.

In some embodiments, the coloration filler is selected from the groupconsisting of TiO₂, Al₂O₃, CaCO₃, CaSO₄, SiO₂, BN, AlN and clay. In oneembodiment, the coloration filler is TiO₂, such as rutile TiO₂, anataseTiO₂, or brookite TiO₂.

The weight ratio for the filler can be between about 5% and about 70% ofthe total weight of the polyimide material, such as between about 10%and about 50%, for example between about 15% and about 30%.

In some embodiments, the white polyimide film can further comprise apigment additive exhibiting complementary color to the polyimide basepolymer and filler. The pigment additive can be a heat resistant bluepigment including, without limitation, organic blue pigments andinorganic blue pigments. In one embodiment, the pigment can be cyanineblue, ultramarine blue, or both. The weight ratio of the blue pigmentadditive can be between about 0% and about 20% of the total weight ofthe polyimide (PI) material, such as between about 0% and about 10%, forexample between about 0% and about 5%.

The white polyimide film is synthesized by a condensation polymerizationreaction involving the reaction of a diamine monomer with a dianhydridemonomer. The molar ratio of the dianhydride to diamine monomers isapproximately 1:1, for example about 0.90:1.10 or 0.98:1.02.

In step 11 of FIG. 1, monomers comprising diamine and dianhydridecomponents are first reacted together in solvent during a condensationpolymerization to obtain a polyamic acid solution. Diamine componentscan comprise TFMB, and dianhydride components can comprise BPDA andBPADA. The solvent can be a non-protonic polar solvent. For economicconsiderations and easy operation, the solvent can have a relatively lowboiling point (e.g., below about 225° C.), so that the solvent can beremoved from the polyimide film at a relatively low temperature.

Examples of suitable solvents can include, without limitation,dimethylacetamide (DMAC), diethylacetamide, N,N′-dimethylformamide(DMF), N,N′-diethylformamide, N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), tetramethylsulfone N,N′-dimethyl-N,N′-propylene urea(DMPU), tetramethyl urea (TMU), N-methylcarpolactam,hexamethylphosphoramide, m-cresol, phenol, p-chlorophenol,2-chloro-4-hydroxytoluene, diglyme, triglyme, tetraglyme, dioxane,dioxolane, cyclohexanone, cylcopentanone, and butyrolactone, and anycombination thereof. In one embodiment, the solvent is DMAC.

In step 12 of FIG. 1, a dehydrant, a catalyst and a coloration fillercan be then incorporated into the polyamic acid solution which isagitated to obtain a homogeneous precursor solution. Examples of thecoloration filler can include TiO₂, Al₂O₃, CaCO₃, CaSO₄, SiO₂, BN, AlNand clay and the like. The weight ratio for the coloration filler can bebetween about 5% and about 70% per PI total weight. The dehydrant can beselected from aliphatic acid anhydrides, such as acetic anhydride andpropionic anhydride, and aromatic acid anhydrides, such as benzoicanhydride and phthalic anhydride, used alone or in combination. In oneembodiment, the dehydrant is acetic anhydride, and the used amount isbetween about 2 and about 3 moles per equivalent of the polyamic acid.

The catalyst can be selected from heterocyclic tertiary amines, such aspicoline, pyridine, lutidine, isoquinoline, quinoline, cinnoline,quinozaline, phthalazine, quinzoline, imidazole, N-methylpyrolidone,N-ethylpyrrolidone, N-methylpiperidine, N-ethylpiperidine, and the like,aliphatic tertiary amines, such as triethylamine (TEA), tripropylamine,tributylamine, triethanolamine, N,N-dimethylethanolamine,N,N-dimethylethanolamine, triethylenediamine, and diisopropylethylamine(DIPEA), and aromatic tertiary amines, such as dimethylaniline, usedalone or in combination.

In one embodiment, the catalyst is picoline, such as α-picoline,β-picoline, or γ-picoline. The polyamic acid:dehydrant:catalyst molarratio can be about 1:2:1, wherein for every mole of polyamic acid, thereis about 2 moles of dehydrant and about 1 mole of catalyst.

In step 13 of FIG. 1, a layer of the precursor solution is coated on aglass or stainless steel plate support.

In step 14 of FIG. 1, the coated layer is baked to form a whitepolyimide film, which can be then peeled from the plate support. Asuitable temperature range for baking is between about 90° C. and about350° C. The white polyimide film thereby formed includes a colorationfiller that is homogeneously distributed in the polyimide base polymerof the white polyimide film.

In some embodiments, step 12 can further comprise incorporating apigment additive presenting complementary color to the color of thepolyimide and the coloration filler into the polyamic acid solution. Thepigment additive can include a heat-resistant blue pigment additive.Suitable blue pigment additives can include, without limitation, organicblue pigments and inorganic blue pigments. More specifically, a bluepigment can be cyanine blue, ultramarine blue and combinations thereof,and the used amount can be between 0 wt % to about 20 wt % per PI totalweight.

Other additives can also be incorporated into the solution containingpolyamic acid in step 12 to confer desired properties to the polyimidefilm. For example, suitable additives can include, without limitation,processing aid, antioxidant, light stabilizer, light extinctioncoefficient modifier, flame retardant additive, anti-static agent, heatstabilizer, ultraviolet light absorbing agent and reinforcing agent,which can be used alone or in combination.

The white polyimide film formed can have a thickness between about 5 μmand about 150 μm, such as between about 5 μm and about 75 μm, forexample between about 10 μm and about 50 μm.

An advantage of the white polyimide film described herein is itsenhanced mechanical properties and resistance to heat. The whitepolyimide film has a coefficient of thermal expansion (CTE) much lessthan 60 ppm/° C. in a temperature range between about 100° C. and about200° C. In particular, it is observed that the CTE of the whitepolyimide film can exhibit enhanced resistance to heat with a CTE lowerthan 45 ppm/° C., such as 30 ppm/° C., more specifically even lower than25 ppm/° C. Moreover, the formed white polyimide film has an elongationrate between about 5% and about 60%, more particularly within a rangebetween about 5% and about 30%.

In addition to the above properties, the formed white polyimide filmalso exhibits desirable color with a L*-value more than about 90, andwith a b*-value between about 0 and about 15, in particular betweenabout 1 and about 10, and as low as between about 1 and about 5. TheL*-value and the b*-value are indexes defined in the conventional “Labcolor space”. The L*-value represents color lightness and the b*-valuecharacterizes the color dimension between yellow and blue.

In the white polyimide described herein, the color characteristicsremain relatively stable, even when the white polyimide film issubjected to heat (e.g., when it is baked), with a variation Δb* of theb*-value that is less than about 5 within a temperature range between250° C. and 320° C., typical baking conditions. The variation is Δb* ofthe b*-value more particularly between about 0.1 and about 1. The whitepolyimide described herein also provides the advantage of a totaltransparency (TT) value that is less than about 40%, such as less than30%, for examples less than 20%, less than 10%, or less than 5%.

Further, for comparison, a film merely composed of the polyimidecomponents represented by the formula (I) and the formula (II) withoutfiller has a b* value between about 1 and about 20.

EXAMPLE 1

Polymerization: Under ambient temperature, nitrogen gas is fed into a500 ml three-necked flask polymerization vessel. All reactions areperformed under a nitrogen environment. About 160 g of dimethylacetamide(DMAC) used as solvent is added into the flask. About 18.77 g (0.059mole) of 2,2′-bis(trifluoromethyl)benzidine (TMFB) are dissolved intothe DMAC. After the TMFB is completely dissolved in the DMAC, about13.35 g (0.045 moles) of biphenyl tetracarboxylic dianhydride (BPDA) andabout 7.87 g (0.015 moles) of 2,2-bis[4-(dicarboxyphenoxy)phenyl]propanedianhydride (BPADA) are added into the solution, which is continuouslyagitated for about 4 hours. A polyamic acid (PAA) solution is formedwith a viscosity of about 150,000 centipoise (cP).

Making Film: The PAA composition is mixed with a dehydrant aceticanhydride, a catalyst picoline, and TiO₂ as white filler to obtain aprecursor solution. The PAA:acetic anhydride:picoline molar ratio isabout 1:2:1. The white filler is incorporated in a ratio of about 3.6 gof TiO₂ for about 60 g of the PAA solution. The precursor solution isagitated for about 2 hours to homogeneously distribute the white fillerinto the PAA solution. Then, a layer of the precursor solution is coatedonto a glass plate support by using a coating blade. The coated layer,having a thickness of about 1 mil (25 μm), is baked in a furnace in astepwise manner at about 100° C. for 30 minutes, then about 200° C. for30 minutes, and subsequently about 300° C. for 30 minutes. A whitepolyimide film thereby formed having a TiO₂/PI weight percentage ofabout 30% can be peeled from the plate support.

EXAMPLE 2

A polyimide film is prepared as in Example 1 except that the PAAsolution is mixed with TiO₂ as filler and a blue pigment additive in aratio of about 2.4 g of TiO₂ and about 0.6 g of the blue pigmentadditive for about 60 g of the PAA solution, and then agitated for about2 hours for homogeneously distributing the filler and blue pigmentadditive in the PAA solution. The composition including the PAA, fillerand blue pigment additive is then casted on a plate support and baked ina same manner as Example 1 for obtaining a white polyimide film. Thepolyimide film thereby formed has TiO₂/PI and blue pigment/PI weightpercentages of about 20% and about 5%, respectively.

EXAMPLE 3

A polyimide film is prepared as in Example 1 except that the PAAsolution, filler and blue pigment additive are mixed in a ratio of about3.6 g of TiO₂ and about 0.6 g of blue pigment additive for about 60 g ofthe PAA solution, and then agitated for about 2 hours to homogeneouslydistributed the filler and blue pigment additive into the PAA solution.The composition including the PAA, filler and blue pigment is casted ona plate support and baked in a same manner as Example 1 to obtain awhite polyimide film. The polyimide film thereby formed has TiO₂/PI andblue pigment/PI weight percentages of about 30% and about 5%,respectively.

COMPARATIVE EXAMPLE 1

Under ambient temperature, nitrogen gas is fed into a 500-mlthree-necked flask polymerization vessel. All reactions are performedunder a nitrogen environment. About 160 g of DMAC used as solvent isadded into the flask. About 18.77 g (0.059 mole) of TMFB are dissolvedinto the DMAC. After the TMFB is completely dissolved in the DMACsolvent, about 13.35 g (0.045 moles) of BPDA and about 7.87 g (0.015moles) of BPADA are added into the liquid solution, which is thencontinuously agitated for about 4 hours for forming a PAA solution witha viscosity of about 150,000 cP. The PAA solution is mixed with adehydrant acetic anhydride and a catalyst picoline with a PAA:aceticanhydride:picoline molar ratio of about 1:2:1 to obtain a precursorsolution. A polyimide film is casted in the same manner as Example 1.

COMPARATIVE EXAMPLE 2

A white polyimide film is formed by mixing a conventional polyimidecomponent with a white filler. More specifically, under ambienttemperature, nitrogen gas can be fed into a 500-ml three-necked flaskpolymerization vessel. All reactions are performed under a nitrogenenvironment. About 160 g of DMAC, used solvent during thepolymerization, is added into the flask. About 19.14 g (0.096 moles) of4,4′-oxydianiline (4,4′-ODA) are added into the flask. After the4,4′-ODA is completely dissolved in the DMAC, about 20.86 g (0.096moles) of pyromellitic dianhydride (PMDA) are incorporated into thesolvent solution. This solution is then continuously agitated for about4 hours for obtaining a PAA solution having a viscosity of about 150,000cP. A TiO₂ white filler is mixed with the PAA solution in a ratio ofabout 3.6 g of TiO₂ for about 60 g of the PAA solution. Finally, apolyimide film is casted in the same manner as Example 1.

Polyimide films formed according to Examples 1, 2, 3 and ComparativeExamples 1 and 2 are respectively subjected to various tests fordetermining certain mechanical properties, color properties of thepolyimide films. The results of these tests are shown in the followingTables 1 and 2.

TABLE 1 Mechanical properties and CTE. Blue pig- Modulus Elon- CTETiO₂/PI ment/PI (Kgf/ gation (100~200° C.) Item (wt %) (wt %) mm²) (%)(ppm/° C.) Example 1 30 0 552 10.5 22.6 Example 2 20 5 548 11.3 22.5Example 3 30 5 575 8 22.3 Comparative 0 0 457 14.3 24 Example 1Comparative 30 0 352 60 28 Example 2

Table 1 shows the Young's modulus (expressed in Kgf/mm²), elongationrate and CTE (expressed in %) of the polyimide film. The mechanicalelongation rate was measured according to the ASTM 882 standard test byusing an universal tensile strength tester. The CTE was measured byusing a thermal mechanical analyzer TMA 2940 (sold by TA Instruments,Inc.). The film expansion was measured by applying a standard load force(e.g., about 0.05 N) under a thermal stress varying from about 100° C.to about 200° C., with a ramping rate of about 10° C./min. As shown inTable 1, white polyimide films obtained with Examples 1 through 3 havebetter mechanical properties in high temperature environment,characterized by relatively lower CTE values.

TABLE 2 Colorimetric properties. TiO₂/PI Blue pigment PI Colorimetrictest Item (wt %) (wt %) L* b* Example 1 30 0 92.4 3.1 Example 2 20 592.7 2.4 Example 3 30 5 93.3 1.9 Comparative 0 0 88.2 4.6 Example 1Comparative 30 0 87 47.8 Example 2

Table 2 shows color characteristics of the polyimide films measured byusing a spectral colorimeter at a temperature of about 25° C. The colorcharacteristics are expressed in the “Lab color space”, wherein theL*-value characterizes the color lightness, and the b*-valuecharacterizes a color dimension from blue to yellow. White polyimidefilms obtained with Examples 1, 2 and 3 have a b*-value that is lowerthan a polyimide film obtained with Comparative Examples 1 and 2. Thistest shows that the white polyimide films formed according to Examples1, 2 and 3 exhibit lower yellowing coloration. Moreover, the polyimidefilms obtained with Examples 1, 2 and 3 have L*-values that are higherthan those of Comparative Example 1 and 2, which indicate higher whitetint.

In the fabrication of LED or LCD devices, the polyimide film istypically subject to a baking step to achieve adhesion of the polyimidefilm. For illustrating stable color characteristics of the polyimidefilm described herein, Table 3 exemplary shows how the b*-value of thepolyimide film obtained with Example 3 may vary under different bakingtemperatures.

TABLE 3 Variation of b*-value for Example 3 Item Baking condition b* Δb*Example 3 250° C./30 min 1.9 — 280° C./30 min 2.0 0.1 300° C./30 min 2.30.4 320° C./30 min 2.5 0.6

The polyimide film formed according to Example 3 has a relatively lowvariation of the b*-value when it undergoes baking. As a result, thepolyimide film described herein can have stable color characteristicsunder thermal stress.

Accordingly, advantages of the white polyimide films described hereininclude excellent mechanical properties, good resistance to heat, andstable chromaticity characteristic under thermal stress.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims and its equivalent systems and methods.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a”, “an”, and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, components, groups, orcombinations thereof, but do not preclude the presence or addition ofone or more other features, integers, steps, operations, elements,components, or groups thereof The method steps, processes, andoperations described herein are not to be construed as necessarilyrequiring their performance in the particular order discussed orillustrated, unless specifically identified as an order of performance.It is also to be understood that additional or alternative steps may beemployed.

1. A white polyimide film comprising: a polyimide base polymer obtainedby reacting diamine with dianhydride components, a coloration fillerhomogeneously distributed in the white polyimide film, and a pigmentexhibiting complementary color to the color of the polyimide basepolymer, the polyimide base polymer consisting of structures representedby the following formulae (I) and (II):

wherein the ratio of m: n is about 0.95-0.05: 0.05-0.95; and thepolyimide film having a total transparency value less than about 40%, acolor defined with a b*-value representing a position of the colorbetween yellow and blue that is between about 0 and about 15, and avariation Δb* of the b*-value between about 0 and about 5 in atemperature range between about 250° C. and about 320° C.
 2. The whitepolyimide film of claim 1, wherein the diamine component is2,2′-bis(trifluoromethyl)benzidine.
 3. The white polyimide film of claim1, wherein the dianhydride is selected from the group consisting of3,3′,4,4′-biphenyltetracarboxylic dianhydride and2,2-bis[4-(3,4-dicarboxyphenoxy) phenyl]propane dianhydride.
 4. Thewhite polyimide film of claim 1, wherein the weight ratio for the filleris between 5% and 70% per polyimide total weight.
 5. A white polyimidefilm comprising: a polyimide base polymer obtained by reacting diaminewith dianhydride components, a coloration filler homogeneouslydistributed in the white polyimide film, and a pigment exhibitingcomplementary color to the color of the polyimide base polymer, thepolyimide base polymer consisting of structures represented by thefollowing formulae (I) and (II):

wherein the ratio of m: n is about 0.95-0.05: 0.05-0.95; and the weightratio for the filler is between 5% and 70% per polyimide total weight,the polyimide film having a total transparency value less than about40%, and a color defined with a b*-value representing a position of thecolor between yellow and blue that is between about 0 and about 15, avariation Δb* of the b*-value being between about 0 and about 5 in atemperature range between about 250° C. and about 320° C.
 6. The whitepolyimide film of claim 5, wherein the diamine component is2,2′-bis(trifluoromethyl)benzidine.
 7. The white polyimide film of claim5, wherein the dianhydride is selected from the group consisting of3,3′,4,4′-biphenyltetracarboxylic dianhydride and2,2-bis[4-(3,4-dicarboxyphenoxy) phenyl]propane dianhydride.
 8. Thewhite polyimide film of claim 5, wherein the coloration filler isselected from the group consisting of TiO₂, Al₂O₃, CaCO₃, CaSO₄, SiO₂,BN, AlN and clay.
 9. The white polyimide film of claim 5, wherein thecolor is further defined with a L*-value representing a lightness of thecolor more than about
 90. 10. The white polyimide film of claim 5,having a coefficient of thermal expansion (CTE) less than about 60 ppm/°C. in a temperature range between about 100 and 200° C., and anelongation rate between about 5 and 60%.
 11. The white polyimide film ofclaim 5, wherein the b*-value is as low as between about 1 and about 5.